1. Field of the Invention
This invention is directed to a composition and method for preparing phenolic foams having improved thermal insulation properties. The composition and method are especially useful in preparing phenolic foam having cell walls which are substantially free of perforations. The invention is also directed to phenolic foams prepared using the composition and method.
2. Prior Art
Phenolic foams prepared from phenol formaldehyde resoles have been known for many years. It is generally agreed that phenolic foams have the best fire rating of any known foam insulation. Phenolic foams do not burn even when contacted by the flame of a blow torch and give off minimal amounts of toxic gases. Phenolic foams can stand temperatures of 375.degree. F. without serious degradation. Phenolic foams have an ASTM E-84 Steiner Tunnel Flame Spread rating of about 5, a Fuel Contribution of about 0 and a Smoke Rating of about 5.
Despite these advantages and generally favorable economics, phenolic foams have not heretofore penetrated the thermal insulation market. One of the main reasons phenolic foams have not been successful is that phenolic foams heretofore have exhibited unsatisfactory initial thermal conductivity or an undesirable increase in thermal conductivity over time. Additionally, the compressive strength of prior art phenolic foam is not as high as desirable for normal handling. It has also been reported that prior art phenolic foams have serious problems with friability and punking.
The general composition and method for preparing phenolic foam are well known. Generally, a foamable phenolic resole composition is prepared by admixing aqueous phenolic resole, blowing agent, surfactant, optional additives and an acid curing agent into a substantially uniform mixture. The curing catalyst is added in amounts sufficient to initiate the curing reaction which is highly exothermic. The exotherm of the curing reaction vaporizes and expands the blowing agent thereby foaming the composition. The foaming process is preferably perfonmed in a substantially closed mold.
The general method for the continuous manufacture of phenolic foam insulation board is as follows. The foamable phenolic resole composition is prepared by continuously feeding into a suitable mixing device the aqueous phenolic resole, blowing agent, surfactant, optional additives, and acid curing catalyst. The ratio of these ingredients is varied depending on the density, thickness, etc. desired in the final product. The mixing device combines these ingredients into a substantially uniform mixture which is continuously applied evenly onto a moving substrate, usually a protective covering such as cardboard, which adheres to the foam. The foaming composition is usually covered with another protective covering such as cardboard which becomes adhered to the phenolic foam. The covered foaming composition is then passed into a double belt press type apparatus where the curing exothenm continues to vaporize and expand the blowing agent, thereby foaming the composition as it is cured.
As mentioned, one of the main drawbacks of prior art phenolic foam is an unsatisfactory initial thermal conductivity (k value). It is believed that one of the main causes of phenolic foam having a poor initial thenmal conductivity is due to the rupturing of the cell walls during the foaning and early curing of the foamable phenolic resole composition. This rupturing causes an immediate loss of fluorocarbon blowing agent which results in a poor initial thermal conductivity. Ruptured cell walls also readily absorb water, causing a further increase in thermal conductivity. It is also believed that ruptured cell walls deleteriously affect the compressive strength and other properties of the phenolic foams. Another main cause of initial poor thermal conductivity in phenolic foams is the loss of fluorocarbon blowing agent before the cell walls of the foaming compositions are sufficiently formed to entrap the blowing agent.
In copending applications, there are disclosed several methods for preventing the rupturing of cell walls during foaming and the loss of blowing agent before the cell walls are formed strong enough to entrap the blowing agent. These methods comprise foaming and curing the foamable phenolic resole composition while maintaining pressure on the foaming mixture and using a phenol formaldehyde resin having certain molecular weight characteristics.
Also as mentioned, another drawback of prior art phenolic foams is the undesirable increase of thermal conductivity over time (k factor drift). Even in the prior art foams which have cell walls which are not ruptured and which have the fluorocarbon entrapped in the cells, the phenolic foams have a tendency to lose the fluorocarbon blowing agent over time with a corresponding increase in thermal conductivity. It is believed that there are two main causes of the increase in thermal conductivity over time.
One cause of the increase of thermal conductivity over time is cracking of the cell walls. In many prior art phenolic foams the cell walls are very thin. When phenolic foams having thin cell walls are subject to high temperatures, the cell walls dry out and crack. Also, since thermal insulation is normally subject to heating and cooling cycles with concomitant expansion and contractions, the cracking of the thin cell walls is aggravated. Cracking of the cell walls allows the fluorocarbon blowing agent to leak out with a loss of thermal insulation and an increase in thermal conductivity.
In our copending application, there is disclosed a means for preventing the cracking of the thin cell walls. This was accomplished by using a phenolic resole having certain molecular weight characteristics which make it possible to produce phenolic foam having cell walls thick enough to withstand thermal and mechanical stresses without cracking.
The main cause of the increase of thermal conductivity over time is the presence of small perforations or pinholes in the cell walls. These small perforations allow the fluorocarbon blowing agent to diffuse out over time and be replaced by air. This slow replacement by air causes an increase in thermal conductivity and loss of thermal insulation. The small perforations also allow the phenolic foam to absorb water, thereby further increasing the thermal conductivity.
In accordance with the present invention, it has been found that perforations in the cell walls are caused by the presence of water in the foamable phenolic resole composition, particularly water that is present in the acid curing catalyst. Accordingly, it is the object of this invention to provide a composition and method of making phenolic foam in which the cell walls are substantially free of perforations.
Another object of this invention is to provide a composition and method for making phenolic foam which does not lose its thermal insulation properties over time.
Another object of this invention is to provide a phenolic foam that has cell walls which are substantially free of perforations.
Additional objects and advantages of the present invention will be apparent to those skilled in the art by reference to the following description and drawings.